Gold Monolayer Research Articles

Synthesis of Monolayer Gold Nanorings Sandwich Film and Its Higher Surface-Enhanced Raman Scattering Intensity.

Most previous studies relating to surface-enhanced Raman spectroscopy (SERS) signal enhancement were focused on the interaction between the light and the substrate in the x-y axis. 3D SERS substrates reported in the most of previous papers could contribute partial SERS enhancement via z axis, but the increases of the surface area were the main target for those reports. However, the z axis is also useful in achieving improved SERS intensity. In this work, hot spots along the z axis were specifically created in a sandwich nanofilm. Sandwich nanofilms were prepared with self-assembly and Langmuir-Blodgett techniques, and comprised of monolayer Au nanorings sandwiched between bottom Ag mirror and top Ag cover films. Monolayer Au nanorings were formed by self-assembly at the interface of water and hexane, followed by Langmuir-Blodgett transfer to a substrate with sputtered Ag mirror film. Their hollow property allows the light transmitted through a cover film. The use of a Ag cover layer of tens nanometers in thickness was critical, which allowed light access to the middle Au nanorings and the bottom Ag mirror, resulting in more plasmonic resonance and coupling along perpendicular interfaces (z-axis). The as-designed sandwich nanofilms could achieve an overall ~8 times SERS signals amplification compared to only the Au nanorings layer, which was principally attributed to enhanced electromagnetic fields along the created z-axis. Theoretical simulations based on finite-difference time-domain (FDTD) method showed consistent results with the experimental ones. This study points out a new direction to enhance the SERS intensity by involving more hot spots in z-axis in a designer nanostructure for high-performance molecular recognition and detection.

Au/Ta(110) and Au/Nb(110) as Highly Active, Stable, and Inexpensive Catalysts for Oxygen Reduction Reaction on Hydrogen Fuel Cell Cathodes: Prediction from First Principles

AbstractWe propose here structures – a gold monolayer on early transition metal surfaces – as a promising alternative to the prohibitively expensive platinum‐based electro‐catalysts for the oxygen reduction reaction (ORR), which occurs on hydrogen fuel cell cathodes. Based on existing knowledge and educated guesses, we preselect Au/Nb, Au/Ta, Au/Mo, and Au/W as potential catalysts. Our calculations show that these materials are stable and have very high dissolution potentials. A deviation from the linear scaling for the binding energy of ORR intermediates, revealed for Au/Nb and Au/Ta, has a desired impact on the ORR thermodynamics. We find it to be a result of hybridization of p‐electronic states of oxygen belonging the OOH‐radical and Au‐pz states. The calculated ORR free energy diagrams show that the ORR onset potentials for Au/Nb and Au/Ta are significantly higher than that for Pt. We thus predict two inexpensive, thermodynamically and electrochemically stable, and highly active ORR catalysts.

Light welding Au nanoparticles assembled at water-air interface for monolayered nanoporous gold films with tunable electrocatalytic activity

Nanoporous gold film (NPGF) has shown great potential for photonics, catalysis, sensing, and renewable energy. Herein, we report a monolayered NPGF with tunable electrocatalytic activity for improved methanol electrooxidation and H2O2 detection. The NPGFs were fabricated via interfacial self-assembly of Au nanoparticles (NPs) with different sizes and a subsequent light welding process. Amongst the NPGFs made from 16, 30, 55, and 70 nm Au NPs (denoted as x nm-NPGF, where x represents the size for the Au NPs), the 16 nm-NPGF shows the highest electrocatalytic activity, which is attributed to the synergistic contribution from its largest surface area, high conductivity, and the smallest hyperboloid-like ligaments that expose more high-index facets. Consequently, the optimized 16 nm-NPGF delivers a large current density of 151 μA cm−2 (at 0.22 V vs. saturated calomel electrode in 0.1 M KOH and 1.0 M methanol) for methanol electrooxidation. Moreover, it as well shows a good linear range (from 0.005 to 45 mM) for H2O2 detection, reaching a detection limit of 2.56 μM and sensitivity of 156.6 μA mM−1 cm−2. Our studies, thus, offer a low-temperature, solution-processable approach to fabricate monolayered NPGF from easily available Au NPs, without harsh dealloying process.

Strong plasmon-exciton coupling between lithographically defined single metal nanoparticles and monolayer WSe2.

Strong coupling between surface plasmons and excitons leads to the formation of plexcitons with new energy states, providing a versatile platform for a range of frontier research subjects. Single plasmonic nanoparticles have recently attracted much attention for realizing strong coupling due to their small mode volume. However, the usually used chemically synthesized metal nanoparticles are unfavorable for accurately tailoring the surface plasmon resonances and adverse to the implementation of on-chip integration. Here, we report for the first time the realization of strong coupling between monolayer WSe2 and lithographically defined single metal nanoparticles. Focusing on gold nanobowties, the large Rabi splitting of 187 meV is achieved. The excitons around the nanogaps in the nanobowties contribute dominantly to the coupling strength, and the coupling strength is larger for smaller nanobowties due to the smaller mode volume. Moreover, the hybrid systems of monolayer WSe2 and gold nanoparticle monomers of nanorods, nanotriangles, and nanodisks are found to closely satisfy the criterion of strong coupling. The strong plasmon-exciton coupling realized by single plasmonic nanostructures fabricated by advanced nanofabrication techniques and monolayer semiconductors can provide new opportunities for manipulating strong light-matter interactions at the nanoscale and facilitate the development of plexciton-based nanodevices with ultrasmall footprints.

Reversible MoS2 Origami with Spatially Resolved and Reconfigurable Photosensitivity.

Two-dimensional layered materials (2DLMs) have been extensively studied in a variety of planar optoelectronic devices. Three-dimensional (3D) optoelectronic structures offer unique advantages including omnidirectional responses, multipolar detection, and enhanced light-matter interactions. However, there has been limited success in transforming monolayer 2DLMs into reconfigurable 3D optoelectronic devices due to challenges in microfabrication and integration of these materials in truly 3D geometries. Here, we report an origami-inspired self-folding approach to reversibly transform monolayer molybdenum disulfide (MoS2) into functional 3D optoelectronic devices. We pattern and integrate monolayer MoS2 and gold (Au) onto differentially photo-cross-linked thin polymer (SU8) films. The devices reversibly self-fold due to swelling gradients in the SU8 films upon solvent exchange. We fabricate a wide variety of optically active 3D MoS2 microstructures including pyramids, cubes, flowers, dodecahedra, and Miura-oris, and we simulate the self-folding mechanism using a coarse-grained mechanics model. Using finite-difference time-domain (FDTD) simulation and optoelectronic characterization, we demonstrate that the 3D self-folded MoS2 structures show enhanced light interaction and are capable of angle-resolved photodetection. Importantly, the structures are also reversibly reconfigurable upon solvent exchange with high tunability in the optical detection area. Our approach provides a versatile strategy to reversibly configure 2D materials in 3D optoelectronic devices of broad relevance to flexible and wearable electronics, biosensing, and robotics.

Synthesis and Catalytic Activities of Metal Shells (Monolayer, Bilayer, and Alloy Layer)-Coated Gold Octahedra toward Catalytic Reduction of Nitroaromatics

Different types of metal shells such as monolayer, bilayer, and alloy layer of gold (Au) and/or silver (Ag) on gold octahedra (AuOh) nanoparticles were synthesized using N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TPDT) without employing any external reducing agents. TPDT, an alternative to sodium borohydride, was used to prepare 15 different core–shell metal nanostructures. The direct colloidal synthesis and stabilization of silver octahedra nanoparticles is still a challenging task. The proposed methodology is an alternative synthetic route for Ag or Au nanoshell coated onto the AuOh core. UV–vis absorption spectroscopy, high-resolution electron transmission microscopy analyses with selected area electron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, and X-ray diffraction techniques were used to characterize the prepared core–shell octahedra nanostructures. High-angle annular dark field scanning transmission electron microscopy–energy-dispersive X-...

Postdeposition UV-Ozone Treatment: An Enabling Technique to Enhance the Direct Adhesion of Gold Thin Films to Oxidized Silicon.

We found that continuous films of gold (Au) on oxidized silicon (SiO2) substrates, upon treatment with ultraviolet (UV)-ozone, exhibit strong adhesion to the SiO2 support. Importantly, the enhancement is independent of micro- or nanostructuring of such nanometer-thick films. Deposition of a second Au layer on top of the pretreated Au layer makes the adhesion stable for at least 5 months in environmental air. Using this treatment method enables us to large-scale fabricate various SiO2-supported Au structures at various thicknesses with dimensions spanning from a few hundreds of nanometers to a few micrometers, without the use of additional adhesion layers. We explain the observed adhesion improvement as polarization-induced increased strength of Auδ−Siδ+ bonds at the Au–SiO2 interface due to the formation of a gold oxide monolayer on the Au surface by the UV-ozone treatment. Our simple and enabling method thus provides opportunities for patterning Au micro/nanostructures on SiO2 substrates without an intermediate metallic adhesion layer, which is critical for biosensing and nanophotonic applications.

Dark-Exciton-Mediated Fano Resonance from a Single Gold Nanostructure on Monolayer WS2 at Room Temperature.

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides with strong many-body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles and monolayer WS2 is explored. A narrow Fano resonance is observed when the hybrid system is surrounded by water, and the narrowing of the spectral Fano linewidth is attributed to the plasmon-enhanced decay of dark K-K excitons. These results reveal that dark excitons in monolayer WS2 can strongly modify Fano resonances in hybrid plasmon-exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.

Tunable Aminooxy-Functionalized Monolayer-Protected Gold Clusters for Non-Polar or Aqueous Oximation Reactions.

Aminooxy (-ONH2) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy 'click chemistry' on the surface of gold nanoparticles. We present here a trifunctional amine-containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer protected clusters (Au MPCs). Diethanolamine is readily transformed into an organic-soluble aminooxy thiol (AOT) ligand using a short synthetic path. The synthesized AOT ligand was coated on ≤ 2 nm diameter hexanethiolate (C6S)-capped Au MPCs using a ligand exchange protocol to afford organic-soluble AOT/C6S (1:1 ratio) Au mixed monolayer protected clusters (MMPCs). This work describes the synthesis of Au(C6S)(AOT) MMPCs and representative oximation reactions with various types of aldehyde-containing molecules, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.

Structural phase transition in surface layer of metals

In the article a review of various phenomena on the surface of pure metals is given: the occurrence of the sur- face states of Tamm and Shockley; quantum size effects; electric double layer, the work function of electrons from a metal. According to modern concepts, the surface layer is a very thin phase that is in thermodynamic equilibrium with volume. A new model of the surface layer of metals has been proposed. The main attention is paid to the structural phase transition in the surface layer. This question is raised by us for the first time. In order to describe phase transitions in nanostructures, various models have been proposed, among which we can mention the Landau mean field method, in which the order parameter is used. We will use the Landau theory, replacing the temperature T with the coordinate h. It was experimentally shown that the heat capacity jumps ΔCp = 0, d = 1.15 (J/mol•K). The molecular dynamics calculations for the heat capacity of gold with particle sizes from 1.5 to 5.5 nm showed that ΔCp ≈ 1.65 (J/mol•K). This is close to our result, given the proximity of computer calculations. The lattice parameter of gold is a = 0.4 nm. So the d (I) layer contains about 3 monolayers of atoms of gold and d (II) — about 6 monolayers of gold.

Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors.

As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal-semiconductor interface at the source and drain electrodes increases, dominating the performance of devices1-3. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors4,5. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide3,5-9. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000±300 ohm micrometres for monolayer MoS2 and 800±200 ohm micrometres for few-layered MoS2. These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS2, enabling high-performance field-effect transistors with a mobility of 167±20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220±50 ohm micrometres on ultrathin niobium disulfide (NbS2) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indiumalloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.

Tailored Combinatorial Microcompartments through the Self-Organization of Microobjects: Assembly, Characterization, and Cell Studies.

A new concept enables the generation of cell microenvironments by microobject assembly at an water/air interface. As the orientation of 30 μm sized polymer cubes and their capillary force assembly are controlled by the surface wettability, which in turn can be modulated by coating the initially exposed surfaces with gold and self-assembled monolayers, unique niches in closely packed arrays of cubes with vertex up orientation can be realized. The random assembly of distinctly different cubes, prefunctionalized or surface-structured exclusively on their top surface, facilitates the parallel generation of different microenvironments in a combinatorial manner, which paves the way to future systematic structure-property relationship studies with cells.

Targeting Protein-Protein Interactions of Tyrosine Phosphatases with Microarrayed Fragment Libraries Displayed on Phosphopeptide Substrate Scaffolds.

Chemical library screening approaches that focus exclusively on catalytic events may overlook unique effects of protein-protein interactions that can be exploited for development of specific inhibitors. Phosphotyrosyl (pTyr) residues embedded in peptide motifs comprise minimal recognition elements that determine the substrate specificity of protein tyrosine phosphatases (PTPases). We incorporated aminooxy-containing amino acid residues into a 7-residue epidermal growth factor receptor (EGFR) derived phosphotyrosine-containing peptide and subjected the peptides to solution-phase oxime diversification by reacting with aldehyde-bearing druglike functionalities. The pTyr residue remained unmodified. The resulting derivatized peptide library was printed in microarrays on nitrocellulose-coated glass surfaces for assessment of PTPase catalytic activity or on gold monolayers for analysis of kinetic interactions by surface plasmon resonance (SPR). Focusing on amino acid positions and chemical features, we first analyzed dephosphorylation of the peptide pTyr residues within the microarrayed library by the human dual-specificity phosphatases (DUSP) DUSP14 and DUSP22, as well as by PTPases from poxviruses (VH1) and Yersinia pestis (YopH). In order to identify the highest affinity oxime motifs, the binding interactions of the most active derivatized phosphopeptides were examined by SPR using noncatalytic PTPase mutants. On the basis of high-affinity oxime fragments identified by the two-step catalytic and SPR-based microarray screens, low-molecular-weight nonphosphate-containing peptides were designed to inhibit PTP catalysis at low micromolar concentrations.

Gold nanoparticle superlattice monolayer with tunable interparticle gap for surface-enhanced Raman spectroscopy.

Taking into account the near-field coupling interaction, non-close-packed two-dimensional nanoparticle (NP) assemblies with centimeter-scale and tunable interparticle gap (d) have attracted considerable attention due to their remarkable physicochemical properties, which show a wide range of potential applications, e.g., surface-enhanced Raman spectroscopy (SERS) active substrates. In the present work, we demonstrate that an Au NP superlattice monolayer (SM) with tunable d, playing a critical role in governing SERS activity, can be created via a rapid liquid-liquid interfacial assembly strategy. We show that the enhancement factor (EF) of SERS has an approximate 1/d2.4 dependence on the gap of Au NP SM assemblies. This work provides a platform for the rational design of plasmon-enhanced spectroscopy active substrates for theoretical studies and for various applications, including SERS-active substrates, photoelectronic devices, biosensors and others.

In situ scattering of single gold nanorod coupling with monolayer transition metal dichalcogenides.

We investigated in situ the interaction between a single gold nanorod and monolayer transition metal dichalcogenides (TMDCs) by atomic force microscopy nanomanipulation and single-particle spectroscopy. We observed that the resonant scattering peak of the hybrid redshifted, the full width at half maximum of the scattering resonance narrowed and the scattering intensity increased compared with those of the same nanorod before coupling with monolayer TMDCs. These results were understood with the aid of finite-difference time-domain simulations, the Fano model, and the classical oscillator model. Also, the spectral features varied with the distance between the nanorod and TMDCs, and the interaction was mainly attributed to the resonant energy transfer effect. Our findings clarify the influence of TMDCs on the plasmonic resonance and contribute to a deeper understanding of the plasmon exciton interaction. These results are beneficial for the optimization of plasmonic nanostructure-TMDC hybrids and their corresponding applications.

Characterization of mixed-ligand shells on gold nanoparticles by transition metal and supramolecular surface probes.

We report herein two methods to characterize the surface of mixed-ligand shell gold nanoparticles, which was explored with gold nanoparticles containing varying molar ratios of 3-mercaptopropionic acid (MPA) and 3-mercapto-1-propanesulfonate (MPS) or 11-mercaptoundecanoic acid (MUA) and triethylene glycol mono-11-mercaptoundecyl ether (TEG) in their ligand shell. Incubation of gold nanoparticles with a solution containing the transition metal cation Ni2+ allows the extraction of Ni2+ depending on the number of negatively charged surface groups and the reaction of surface carboxylic acid groups with an aminomethyladamantane derivative allows the extraction of the supramolecular host molecule cucurbit[7]uril (CB7) depending on the number of reactive surface groups. In both the methods, the remaining surface probes in the supernatant could be conveniently quantified in a homogeneous solution after a simple centrifugation step. An excellent linear correlation between the amount of Ni2+ extracted and the ligand density of MPA and MPS in MPA/MPS gold nanoparticles or MUA in MUA/TEG gold nanoparticles afforded a simple and reliable assay method to determine the number of negatively charged surface groups. The supramolecular CB7 assay enabled the determination of the accessible ligand density of reactive surface carboxylic acid groups and revealed a striking difference in the number of surface groups that can be reacted with MPA/MPS gold nanoparticles or MUA/TEG gold nanoparticles, which suggests a simple method to probe the surface structure of mixed monolayer gold nanoparticles.

Probing Electrolyte Solvents at Solid/Liquid Interface Using Gap-Mode Surface-Enhanced Raman Spectroscopy

Understanding the aprotic solution structures at the immediate vicinity of solid/liquid interface (SLI) is critically important for next generation lithium ion battery development. Yet, it is still challenging to investigate the carbonate chemical profiles close to the diffuse layer (about 10 nm) of the electrical double layer at SLI due to the lack of a ultrahigh surface sensitive tool. In this work, we demonstrate the structures of commonly used carbonate solvents (ethylene carbonate (EC) and diethyl carbonate (DEC)) and a carbonate additive (fluoroethylene carbonate (FEC)) in a commercial Li-ion battery electrolyte can be determined at ∼17 nm above the electrode surface. This is only enabled by a nanogap surface-enhanced Raman spectroscopy (SERS) technique based on a monolayer gold nanoparticle (Au NP) ensemble. The SERS enhancement factor (EF) of those carbonates was found to depend on the molecular polarizability, with the maximum EF at ∼105 found for EC and FEC. Despite their alike chemical structures, this monolayer Au NP SERS substrate is fully capable of discrimiating the different Raman finger prints of EC and FEC. Compared to EC, several vibration modes in FEC, such as C-C skeletal deformation, ring breathing band and C=O stretching band, shift to higher frequencies because of the displacement of a hydrogen atom by a much heavier fluorine atom in a methylene bridge. This counterintuitive observation against the commonly used “ball and spring” model in vibrational spectroscopy is mostly due to the increased bond strength in the FEC ring versus that of EC. A second order empirical polynomial best describes the correlation between the SERS band integration of EC or DEC molar concentration. Our findings open up new opportunities for in-depth understanding of the electrolyte molecular vibrational behaviors at direct solid/liquid interface and developing advanced electrolytes for next generation lithium-ion batteries.

Compact dual-fiber surface-enhanced Raman scattering sensor with monolayer gold nanoparticles self-assembled on optical fiber.

Employing a self-assembly method and dual-fiber structure, a compact, low-cost, highly sensitive, fast surface-enhanced Raman scattering (SERS) optical fiber sensor was realized. Through detecting a SERS signal of 10ppm rhodamine B analyte, sensor parameters such as coupling angle, fiber types, and corresponding glass substrate structure were optimized. The ratio of the SERS signal intensity to excitation light residual peak intensity was used as a significant parameter during the optimization process. Sensor characteristics such as the SERS signal dependence on excitation power, stability related to excitation time, and reusability were studied. Its compatibility in size with microfluidic structure would make it a prospective candidate for integrating into a microfluidic chip.

Gold/silver/gold trilayer films on nanostructured polycarbonate substrates for direct and label-free nanoplasmonic biosensing.

Ultrasmooth gold/silver/gold trilayer nanostructured plasmonic sensors were obtained using commercial Blu-ray optical discs as nanoslits-based flexible polymer substrates. A thin gold film was used as an adhesion and nucleation layer to improve the chemical stability and reduce the surface roughness of the overlying silver film, without increasing ohmic plasmon losses. The structures were physically and optically characterized and compared with nanostructures of single gold layer. Ultrasmooth and chemically stable trilayer nanostructures with a surface roughness <0.5 nm were obtained following a simple and reproducible fabrication process. They showed a figure of merit (FOM) value up to 69.2 RIU-1 which is significantly higher (more than 95%) than the gold monolayer counterpart. Their potential for biosensing was demonstrated by employing the trilayer sensor for the direct and refractometric (label-free) detection of C-reactive protein (CRP) biomarker in undiluted urine achieving a Limit of Detection (LOD) in the pM order.

Light-Controlled Shrinkage of Large-Area Gold Nanoparticle Monolayer Film for Tunable SERS Activity

The two-dimensional (2D) monolayer gold nanoparticle (Au NP) film is of significant interest and importance in both fundamental and practical applications including optoelectronic devices, sensing, catalysis, and surface-enhanced Raman spectroscopy (SERS). Because of the weak physical interaction, the conventional monolayer Au NP film fabricated at the oil–water interface was unstable, easily breakable, and difficultly transferred. In the present work, we report on a simple and effective chemical cross-linking strategy at the air–water interface to achieve a large-scale monolayer gold nanoparticle film with intelligently tunable size of nanogaps, and excellent free-standing and easily transferable properties. In our strategy, acrylamide, a polymerizable molecule, was first modified on the surface of Au NPs for subsequent self-assembly into a monolayer film at the liquid–liquid interface. Through photopolymerization of acrylamide, a chemically cross-linked film was formed with closely packed nanoparticles,...